Mass Wasting and Landslides
Mass Wasting
1 Introduction
Landslide and other ground failures posting substantial damage and loss of life
In U.S., average 25–50 deaths; damage more than $3.5 billion
For convenience, definition of landslide includes all forms of mass-wasting movements
Landslide and subsidence: naturally occurred and affected by human activities
Mass wasting
Downslope movement of rock and soil debris under the influence of gravity Transportation of large masses of rock Very important kind of erosion
2 Mass wasting Gravity is the driving force of all mass wasting
Effects of gravity on a rock lying on a hillslope
3 Boulder on a hillside
Mass Movement
Mass movements occur when the force of gravity exceeds the strength of the slope material
Such an occurrence can be precipitated by slope-weakening events Earthquakes Floods Volcanic Activity Storms/Torrential rain Overloading the strength of the rock
4 Mass Movement
Can be either slow (creep) or fast (landslides, debris flows, etc.) As terrain becomes more mountainous, the hazard increases
In developed nations impacts of mass-wasting or landslides can result in millions of dollars of damage with some deaths
In less developed nations damage is more extensive because of population density, lack of stringent zoning laws, scarcity of information and inadequate preparedness
**We can’t always predict or prevent the occurrence of mass- wasting events, a knowledge of the processes and their relationship to local geology can lead to intelligent planning that will help reduce losses of life and property
Controls and Triggers of Mass Wasting
Important factors include: The role of water Diminishes particle cohesion (friction) Water adds weight Oversteepening of slopes—slope angle Stable slope angle (angle of repose) is different for various materials Oversteepened slopes are unstable
5 Angle of Repose- Effect of Grain Size
Angle of Repose- Effect of Moisture Content
6 How do they do that?
Surface Tension
7 Unsaturated Soil
Saturated Soil
Slope Instability Caused by Oversteepening
8 Water causing failure in slopes of solid bedrock
Water causing failure in slopes of solid bedrock
9 Angle of Repose
S. C. Porter
Controls and Triggers of Mass Wasting
Important factors Removal of anchoring vegetation Root system binds soil and promotes stability Earthquakes as triggers May cause expensive property damage Can cause liquefaction—Water-saturated surface materials behave as fluid-like masses that flow
10 Fire increases the susceptibility for erosion and mass movement
Controls and Triggers of Mass Wasting
Landslides without triggers Slope materials weaken over time. Random events that are unpredictable
11 Weathered Shale
Fig. 11.6 Martin Miller
Classification of Mass Wasting Processes
Generally, each event is classified by: Type of material involved Mud, Earth, Rock Type of motion Fall (free-falling pieces) Slide (material moves along a surface as a coherent mass) Flow (material moves as a chaotic mixture) The velocity of the movement Fast or Slow
12 Mass Wasting Processes
Rockfalls
13 Sunday May 9, 1999 50 people injured 8 people dead
Rockfall was small -25-30 cubic yards -Weathered basalt -500 feet above
14 15 16 Bad Car Day
17 Slump
18 What causes slumps? •Most common reason for slumping • Erosion at the base of the slope • Reduces the support for overlying sediments •Erosion at the base of a slope has many causes • Stream channel, wave action, seepage of water into the ground during the rainy season, etc.
Wave cut wall
Processes that oversteepen slopes
19 Processes that oversteepen slopes
Processes that oversteepen slopes
20 Slopes susceptible to mass movement
21 Debris flows
Occur when the rock/soil mass loses coherency and lots of water is involved Debris becomes mixed up completely and flows as liquid mud Often carry large clasts Can be very destructive
22 Debris flow
23 24 Debris avalanche
25 Debris slide
26 Earthflow
27 Mudflow
28 Soil Creep
Creep Gradual movement of soil and regolith downhill Aided by the alternate expansion and contraction of the surface material Mass movement that moves very slowly Weak soils on steep slopes move slowly down hill
29 Some Visible Effects of Creep
30 Landslides Occur when a large piece of rock and/or soil breaks off and slides down hill Often initiated by earthquakes and by very heavy rainstorms Can occur when humans foolishly overload a road bordering a canyon or steep drop off with heavy equipment
Landslides
Initiated when rock/soil originally held in place by internal cohesion suddenly loses that cohesion Form on slopes that are steep enough for the weight of the surficial material to overcome the cohesive force and fail
31 Steep Headwall
Spoon-shaped sliding surface
32 Jumbled-up Toes
33 Landslides Tendency to slide increases with increasing surface slope Addition of water promotes sliding by adding weight and by reducing cohesion Friction along the slide surface controls the speed of the downslope movement
Landslides
Addition of water reduces friction along the surface and allows the mass to slide faster
Some slides move as coherent mass
Others break up and the material inside becomes jumbled and disorganized
34 If bedding dips down-slope, a landslide is more likely to develop
35 Landslides Certain areas are more prone than others Usually because of local geological factors Reducing vegetation that was stabilizing the soil Over watering (decrease friction)
36 Dangerous OK to cut to cut
Cutting a slope during construction can cause a landslide
37 LaConchita (S. California) landslide, January, 1995
38 LaConchita (S. California) landslide, January, 2005
LaConchita (S. California) landslide, January, 2005
39 Our Islands Are Unstable
From Shoreline to Mountains Even the youngest parts of the volcanoes are subject to landsliding
40 41 Dating mass-movement events
42 Landslide Induced by Poor Construction Techniques
43 Human Land Use and Landslide
Timber harvesting in weak, relatively unstable areas
Artificial fillings of loose materials
Artificial modification of landscape
Dam construction
44 Minimizing the Landslide Hazard
Identifying potential landslides ¾ Photographic analysis ¾ Topographic map and detailed field check ¾ Historic data
Landslide hazard inventory map ¾ Grading code from the least stable to the most stable
Application of geologic and engineering knowledge before any hillside development
Figure 9.16
45 46 Minimizing the Landslide Hazard
Preventing landslides ¾ Drainage control: Reducing infiltration and surface runoff ¾ Slope grading: Reducing the overall slope ¾ Slope supports: Retaining walls or deep supporting piles
Avoid landslide hazards ¾ Landslide warning for critical evacuations ¾ Correcting landslides
47 48 49 Warning of Impending Landslides
Monitoring changes ¾ Human surveillance ¾ Instrumental survey: Tilt meter and geophones
Landslide warning system ¾ Info for public awareness and education ¾ Enough time for public evacuation ¾ Stop or reroute traffic flow ¾ Emergency services
50 What Can You Do?
Professional geologic evaluation for a property on a slope Avoid building at the mouth of a canyon, regardless of its size Consult local agencies for historical records Watch signs of little slides—often precursor for larger ones
What Can You Do?
Look for signs of structure cracks or damage prior to purchase Be wary of pool leaking, tilt of trees and utility poles Look for linear cracks, subsurface water movement Engineering solutions (e.g., retaining walls: to stabilize slopes, drainage pipes: to drain water, grade slopes: to reduce oversteepening and diversion walls: to protect structures) Put observations into perspective, one aspect may not tell the whole story
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